let isDone: boolean = false
isDone = true


let age:number = 10;

let message:string = `hello ${age}`

let u:undefined = undefined

let n:null = null

let num:number = undefined

let str:string = undefined

let b:number = null

let any:any = 'hello'


any = `maybe a string`
let username:string = 'wangyang'


// undefined 和 null 是所有类型的子类型；也就是说：unfettered类型变量 可以赋值 null类型的变量


